1. Field of the Invention
Anti-skid hydraulic brake systems.
2. Prior Art
A pressure control arrangement disclosed in German unexamined and printed publication DT-OS Pat. No. 1,947,012 comprises a plunger piston which is sealingly slidable in a bore which defines a chamber connected with at least one wheel brake cylinder, the chamber being connected with the brake pressure source through a valve which is kept in the open position by a mechanical means in the end position of the plunger piston in which the chamber volume is the smallest. The plunger piston is displaceable into two end positions by pressure supplied by an auxiliary pressure source which is controllable for effecting anti-skid control, for which purpose the auxiliary pressure source is connected through at least one auxiliary valve to a control chamber lying opposite the chamber which is in communication with at least one wheel brake cylinder.
In the known pressure control arrangement a valve member of the valve is biased into the closing position by a spring and is kept open by the plunger piston through a tappet fitted thereto when the plunger piston is in its end position. When the plunger piston leaves its end position the tappet will move away from the member thereby enabling the spring to close the valve. In the event of a pressure decrease in the chamber, the differential pressure thereby occuring exerts an additional pressure on the valve member in the closing direction. The valve can be reopened only after the plunger piston has again reached its end position which thus disregards the fact that the valve performs, at the same time, the function of a check valve inhibiting the passage of fluid in the direction towards the chamber. Because of this purely mechanical valve control, care must be taken to assure that, in the event of failure of the auxiliary pressure which controls the plunger piston, the plunger piston will maintain, in any case, its end position to keep the valve open. Otherwise, the plunger piston would be moved away from its end position upon its being acted on by a slight pressure supplied to the wheel brake cylinders. The valve will be closed thereby and no further brake pressure could be built up. Accordingly, the known pressure control arrangement provides for another auxiliary power source in the form of an emergency spring which is kept in a standby position by the auxiliary pressure for so long as it is available. If the auxiliary pressure fails, the spring will relax keeping the plunger piston in its end position. In this known arrangement the emergency spring must still have a biasing force sufficiently high to enable the plunger piston to maintain its end position in opposition to the maximum possible brake pressure.
It is a serious drawback of this known pressure control arrangement that generally an outside power source has to be provided to assure normal braking without any skid control in those cases where the auxiliary pressure has failed. This results in a device which is large in size, heavier and increased cost for a pressure control arrangement. In addition, the positioning of such a strong prestressed emergency spring causes considerable difficulties. These drawbacks have particular weight because the emergency spring is required only for rarely occurring events as an additional safety measure just to keep the valve open. The fact that it prevents at the same time an increase of the chamber volume in the event of a failure of the auxiliary pressure, which means no pressure fluid is drawn from the brake system is rather irrelevant for a larger number of brake systems since a limited increase of the chamber volume can be handled without difficulty.
An example of the known braking system described above is set forth in German Pat. No. 909,657. In German Pat. No. 909,657 the plunger piston is completely mechanically separated from the valve. The plunger piston is controlled by the auxiliary pressure purely by hydraulic means, with the provision of additional means for keeping it in the end position upon failure of the auxiliary pressure. If the auxiliary pressure fails it will thus be displaced into a position enlarging the chamber volume. The valve will be switched mechanically independent of the plunger piston's position. To prevent the cost of such a device from becoming too high, it is further provided with means to switch the valve together with an auxiliary valve controlling the auxiliary pressure for the anti-skid control by means of a solenoid. In this device the auxiliary valve has the same function as the auxiliary valve described in the pressure control arrangement set forth in DT-OS Pat. No. 1,947,012 which function is to disconnect a control chamber from the outside pressure source and connect it with the return line in order to achieve a decrease in brake pressure. To increase the brake pressure again the control chamber is reconnected with the outside pressure source so that the pressure increases therein and the plunger piston is urged back into its end position.
In the brake system according to German Pat. No. 909,657 the valve opens completely independent of the position of the plunger piston and the brake pressure prevailing in the chamber. Thus there will be no approximation between the effective brake pressure and the pressure of the brake pressure source. To the contrary the valve will reopen just when the brake pressure in the chamber is at its lowest value within the control cycle because it is at this moment that the auxiliary valve has to change its position to urge the plunger position back into its end position. Due to the valve opening at that moment, pressure fluid from the brake pressure source will substantially flow into the chamber which leads to a substantial yielding of the brake pedal. Also the brake pedal will be increased to the pressure level of the brake pedal source before the plunger piston reaches its end position. If the wheel concerned does not become locked again, thereby the plunger piston will continue moving into its end position out of the chamber again thereby causing the brake pedal to be raised again to the opposition of the force being exerted by the driver. In most cases, however, the wheel will lock again due to the sudden increase of pressure so that another control cycle will follow in which the plunger piston starts to leave its middle position. The distance within which the plunger piston is allowed to move must be closely restricted in order to prevent the situation in which the chamber draws too much pressure fluid in the case of failure of the auxiliary pressure which would entail the risk of exhausting the brake pressure source. As a consequence, a movement of the plunger piston away from its middle position to avoid wheel lock is not sufficient. It will be apparent from the above that the mechanical control of the valve depending upon the plunger piston is necessary in a pressure control arrangement of the type initially referred to in order to ensure a satisfactory anti-skid control.
In German Pat. No. 2,130,100 a pressure control arrangement has already become known which comprises substantially a floating piston inserted in the brake line. In the arrangement of this patent, one piston end is exposed to the pressure in the brake pressure source which causes displacement of the piston and generation of the brake pressure acting on the wheel brake cylinder on the other piston end. To achieve an anti-skid control, the auxiliary pressure controlled by auxiliary valves, is supplied to an additional annular surface of the piston acting against the pressure from the brake pressure source which causes the piston to move. While it is true that this pressure control arrangement has the advantage that failure of the auxiliary pressure merely causes the anti-skid control to become ineffective and does not effect the brake system, it is, however, a disadvantage in that any control cycle, no matter how small the decrease in brake pressure it causes is, results in pressure fluid being urged back to the brake pressure source which immediately makes itself felt at the brake pedal in the case where static brake pressure sources are used. In contrast thereto it should be the aim of all anti-skid control systems to achieve, in a normal control cycle, during which only a minor pressure decrease is necessary, no pressure fluid can be urged back. In a strong control cycle effecting substantial pressure changes which usually occurs only during variations of the coefficient of friction, a small amount of pressure fluid which is urged back to the brake pressure source could be withstood without difficulty. In this case, when using a static brake pressure source, a normal control cycle would not make itself felt at all and a strong control cycle could be felt only slightly. Such an operating characteristic would no doubt constitute an advantageous side effect in a pressure control arrangement. However, of primary importance are the disadvantages inherent in the known pressure control arrangements.
The device of this invention is therefore based on the pressure control arrangement described in DT-OS 1,947,012 since that arrangement, apart from the disadvantages described, provides an advantage in that its control cycle does not effect the control brake pressure source.